The incidence of Candida albicans infections is increasing. The mortality rate of hospitalized patients infected with Candida, even with treatment, is unacceptably high (up to 34 percent). Furthermore, treatment with current antifungals is still not optimal (toxicity, resistance development, etc.). Thus there is a need for novel approaches to treat and prevent candidiasis. One of these approaches focuses on exploiting the concept of candidal virulence by identifying and characterizing factors, such as phospholipase B (PLB), that augment the pathogenicity of this important nosocomial pathogen. Such factors may well represent the basis for the development of new and improved therapeutic strategies. In the last granting period we cloned and sequenced caPLB1, the gene encoding PLB, from C. albicans. This enzyme is the dominant phospholipase secreted by this yeast. Using immunoelectron microscopy, we showed that PLB is expressed in vivo during the infectious process. We also showed that sera from patients with systemic candidiasis have anti PLB antibodies. We constructed a genetically defined C. albicans strain pair differing only in the ability to secrete PLB. Testing these strains in two murine models of candidiasis (iv and oral-intragastric) showed that the virulence of PLB1-deficient mutants was reduced compared to the parental strain. These results indicate that PLB is associated with the virulence of C. albicans. This application will expand our previous studies by focusing on the mechanism/s by which PLB modulates candidal virulence. Critically, these mechanistic studies will be facilitated by the availability of the isogenic strain pair and the purified enzyme. These tools will enable us to clearly define the role of PLB in the virulence of C. albicans beyond that which was previously possible. Our specific aims are: 1) Determine the mechanism/s by which PLB augments the virulence of C. albicans. We will explore if PLB influences host-parasite interactions pivotal to the pathogenesis of C. albicans. 2) Investigate the physiology and regulation of PLB expression. Having shown that PLB is expressed in vivo we will determine the physiological conditions which enhance its expression and how it is regulated. 3) Determine the combined contribution of extracellular enzymes, phospholipase and aspartyl proteinase (SAPs), to the virulence of C. albicans. Using specific proteinase inhibitors and mutants deleted for both PLB1 and SAPs, we will determine whether these enzymes act in concert to enhance the virulence of C. albicans.